The present invention is generally covers receivers in wireless communications systems, and more specifically is generally drawn to addressing noise and/or interference effects exhibited by received signals, where the signals were transmitted via a transmitter employing high power amplifiers (HPAs), such as satellite transponders in a satellite communications system. Satellite communication systems must transmit signals over vast distances from earth to satellites in orbit and vice-versa. A communication system may include a transmitter having an HPA or a transponder that includes a transmitter having an HPA. If a transmitter (or transmitter section of a transponder) is located in a space-based satellite, there is limited access thereto. Accordingly, compensating for the distortion associated with an HPA within a space-based satellite transmitter is much more complicated than compensating for the distortion associated with an HPA within a ground-based transmitter. Additionally, satellites in general have strict power consumption limits that require the communication systems to operate at very high efficiencies of both power use and usage of available communication bandwidth.
Due to physical limitations, there is a maximum number of HPA units that can fit in a transponder. Sharing multiple carriers by a single transponder HPA allows for transmitting more data and servicing more users without exceeding this physical limitation. Another benefit of this multicarrier operation is that it allows for reducing the transmission symbol rate per carrier without sacrificing system throughput. This greatly eases the burden on hardware implementation.
However, when multiple carriers are amplified by way of a single HPA, and when driven near its saturation creates a large amount of nonlinear interference. This interference manifests as a large mismatch between what the FEC decoder is expecting and what is received, causing a large degradation in performance.
In essence, many conventional satellite communication systems with HPAs are able to: drive the HPA in or near saturation while efficiently communicating over a single carrier; or inefficiently communicate over a plurality of carriers without driving the HPA in or near saturation. For example, one conventional system addresses the above using a statistical model at the decoder input to compensate for interference. However, such a system ignores the impact of nonlinear interference, resulting in a system that is unable to efficiently communicate over a plurality of carriers in or near saturation.
Other attempts to compensate for nonlinear interference have been complex and require receivers to exchange information. For instance, a conventional system compensates for linear and nonlinear intersymbol interference (ISI) and linear and nonlinear adjacent channel interference (ACI) due to the nonlinearlity of HPA and tight crowding of carriers in a transmitter HPA or transmitter section of a transponder HPA. However, such a system requires receivers to coordinate samples from adjacent carriers, resulting in increased system complexity and computational effort.
What is needed is a method of compensation that addresses nonlinear interference on the FEC decoding operation, particularly when multiple carriers share a single HPA that requires no exchange of information relating to adjacent carriers.